Dilute Solutions of He3 in He4 at Low Temperatures

Abstract

Properties of dilute solutions of He3 in He4 at low temperatures and their application to dilution refrigeration are discussed from a review—tutorial point of view. A comprehensive review of the properties of dilute solutions is not given. Rather, the simple physical notions underlying an understanding of their properties are discussed. Special emphasis is placed on those phenomena which are of importance in understanding the dilution refrigerator. After a review of the phase-separation phenomenon in He3-He4 solutions, the experimental and theoretical basis for the effective interaction between He3 quasiparticles is discussed. Then the energy of the He3 quasiparticles, their chemical potential, and their dependence on the local density and velocity of the superfluid are treated. A few brief remarks on a possible superfluid transition are made. A discussion is given of the qualitative aspects of different types of dilution refrigerators: the “evaporation”-like continuous and single-cycle refrigerators and the “adiabatic expansion”-like superleak operated dilution refrigerator. Then a thermodynamic treatment of these refrigerators is given. Finally some numerical and graphical information, based on the actual properties of He3 and dilute solutions, is given to illustrate the possibilities and problems of the methods.